The invention may be particularly applicable to a moulded boat hull comprising an outer shell and an inner shell.
One generally known method of forming a boat hull from moulded plastic shells it to mould separate outer and inner shells and then to join the two shells together either by using adhesives, or by fastening devices such as bolts, screws and the like. This method of construction enables the internal surfaces of the hull to be inspected for quality and thickness before assembly, and enables various shaped sections of the hull to be joined together where required to obtain the necessary strength. In particular, it enables hollow girder sections to be formed lengthwise along the hull to provide rigidity of the hull. However with conventional methods of attachment of the outer and inner shells using fastening devices there are inevitably stress concentrations in the regions of the joints, and in the case of using adhesives there is the possibility of imperfect jointing also leading to local areas of stress concentration. As a result the joints are susceptible to fatigue failure. Consequently these methods of construction are not satisfactory for hulls of larger high powered vessels subjected to high operational loadings over long periods of time. It is therefore desirable with moulded hulls, to have an integral construction to facilitate even load distribution throughout the structure thereby minimising problems of failure at regions of attachment.
Another drawback with the conventional method of construction is that it involves two processes. The first process being to form the two shells and the second process being to join the two shells together. Consequently the method is time consuming and hence costly.
An alternative method of construction that reduces production time and partially overcomes the problem of joining is the rotary moulding process. With the rotary moulding process a complete shell can be moulded in a single step thereby speeding up production and reducing labour costs. With this method, moulds for outer and inner shells of the hull, such as the hull bottom and stern, and the hull decking, seating and flooring sections respectively are made so that they can be attached together to form a single hollow mould.
In the process, a rotary plastic moulding powder such as Dupont 8405 moulding powder of a predetermined amount is first poured into one of the shell moulds. Then after attaching the moulds together, the resulting hollow mould is heated while being rotated and tilted so that the powder can flow over all internal surfaces of the mould, sticking to the internal surfaces as it melts under heat from the mould and forming into a homogeneous plastic. By programming the sequence and degree of rotation and tilting, the whole of the internal surfaces of the mould can be evenly covered with plastic to a predetermined thickness so that an integral hollow shell is formed. With this method of construction, the peripheral edges of the moulded hull, that were necessarily bonded or fastened together in the conventional two process method, can be formed as homogeneous integral parts of the hull, thereby ensuring even stress distribution in these regions to that fatigue failure is minimized. Furthermore certain other sections such as those around centreboard wells where the bottom surface of the hull is not continuous can also be formed as homogeneous integral parts of the hull connecting the inner and outer shells of the hull together.
With the above rotary moulding technique however, due to the requirement of allowing the plastic to flow in powder form over all the surfaces to be coated with plastic, it has not been possible to form internal sections of homogeneous plastic such as girder sections connecting the inner and outer shells where one of the surfaces of the shell is continuous.
It is an object of the present invention to provide a method of forming a shell structure that addresses the above problems and difficulties or at the very least offers a useful choice.
It is a further object of the present invention to provide a moulded plastic shell formed by such a method.